Step wave power converter

ABSTRACT

A single- or multi-phase step wave power converter includes multiple transformers configured to receive DC voltage from one or more power sources. Each of the transformers includes a primary winding and a secondary winding. The transformers are each configured to supply a step for a step wave AC output. Bridge circuits are supplied for controlling input of DC voltage into the primary windings of the transformers. Steps for the step wave AC output are output from the secondary windings based upon the input provided to the primary windings. DC source management circuitry manages which DC power source(s) supplies DC voltage input to each of the bridge circuits. The management circuitry provides seamless power switching between the plurality of DC power sources based on each power source&#39;s performance characteristics. A pulse-width modulator can also be provided to the step wave power converter to modulate the input into a selected primary winding. In this way, the step wave AC output can be fine-tuned in substantial conformance with an ideal AC waveform.

This application is a continuation of Ser. No. 09/468,610, filed on Dec.21, 1999, now U.S. Pat. No. 6,198,178 which claims the benefit ofProvisional application Ser. No. 60/113,424, filed Dec. 22, 1998.

BACKGROUND OF THE INVENTION

This invention relates generally to step wave power converters fortransforming power from power sources supplying DC voltage input into ACpower. More specifically, this invention relates to step wave powerconverters for providing greater input control over multiple DC powerbuses and for more accurately simulating single- or multiple-phase ACwaveforms. While this invention is particularly directed to thetransformation of power from DC power sources to AC power, it should benoted that AC power sources can be readily converted to DC power sourcesthrough the use of a rectifier. Therefore, the scope of this inventionis not limited to strictly DC-to-AC power conversion.

Prior art patents and publications describe various single-phase stepwave power converters for transforming DC voltage input into a step waveAC output. FIG. 1 is a schematic illustration showing one example of aprior art power converter. Referring to FIG. 1, one single-phase stepwave power converter of the prior art uses one transformer 2 for eachstep of the step wave output. A single DC power source is used to supplypower to each of the transformers 2 in the power converter. Eachtransformer 2 has three windings, including two primary windings P1, P2and one secondary winding S. The two primary windings P1 and P2 areelectrically coupled to the DC power source through four gates G1-G4.The gates G1-G4 control the flow of current through the primary windingsP1, P2 in order to produce a step of the AC output from the secondarywinding S. The two primary windings P1, P2 in each transformer 2 areidentical to each other except that they are oppositely connected to theDC voltage source. Because of their reverse connections, they induceopposite polarity voltage in the secondary winding S. The secondarywindings S of the transformers are connected together in series so thattheir outputs can be combined to produce the step wave AC output.

In operation, the gates G1-G4 are controlled to alternately pulse DCcurrent through the primary windings P1, P2. Current flow through apositive polarity primary winding P1 induces a positive step output fromthe corresponding secondary winding S, while, conversely, the flow ofcurrent through a negative polarity primary winding P2 induces anegative step. Steps from the secondary windings S of all of thetransformers 2 are added together to form the overall AC waveform.Consequently, pulsing DC current through the primary windings P1, P2 atthe appropriate time intervals causes the secondary windings S to outputan approximate AC waveform.

U.S. Pat. No. 5,373,433 issued to Thomas (Thomas) provides animprovement in the art with respect to single-phase power invertersoperating from a single DC power source. Specifically, Thomas disclosesswitching bridges for controlling DC voltage input into multipletransformers from a single source. Each switching bridge includes fourswitches arranged in two parallel lines, each of which has two seriesconnected switches. The switching bridges are controlled so that thetransformers produce either a positive, zero, or negative output voltagestep at a given time. According to Thomas, the transformers “preferablyhave turns ratios that are multiples of each other in order to provideboth good resolution and a wide dynamic range of the [AC output]signal.” Col. 5, 11. 58-62. For example, Thomas explicitly discloses asingle-phase power converter having three transformers capable ofproducing voltage outputs from their secondary windings of ±15V, ±45V,and ±135V, respectively. The output voltages from the secondary windingsof all of the transformers are combined in series. Thomas produces afairly accurate AC waveform by controlling timing and sequencing of thevoltage contributions from the three transformers to transitionsequentially through each of twenty-seven different possible overalloutput voltage levels. A special decoder circuit is also provided toprevent accidental shorting across the DC voltage input which wouldoccur if two switches in a series connected pair were closed at the sametime. Despite its improvements, Thomas does not contemplate either theuse of multiple power sources or three-phase operation.

Another prior art topology is described in U.S. Pat. No. 5,631,820issued to Donnelly et al. (Donnelly). Donnelly provides an improvementin the art by using three gates instead of four to control current flowthrough primary transformer windings. Also, although using transformershaving two primary windings and one secondary winding, Donnelly'sswitching architecture allows each primary winding to be used to produceeither a positive or a negative step, rather than only one or the other.Donnelly also provides an improvement in the art by contemplating theuse of multiple power sources, but fails to provide seamless integrationand management of the multiple power sources based on their performancecharacteristics. Donnelly also discloses a three-phase power convertertopology that has nine gates and one three winding, three-phasetransformer per step.

Other prior art patents and publications also describe three-phase stepwave power converters for converting DC voltage from one or more DCpower sources to a step wave AC output. Referring to FIG. 2, one exampleof a prior art three-phase step wave power converter includes multiplethree-phase transformers 4, each having three windings (two primary P1,P2 and one secondary S) per phase per step. The configuration of eachphase is similar to the single-phase arrangement of the prior artdescribed above with reference to FIG. 1. Each phase of each transformerincludes two primary windings P1, P2 and a secondary winding S. The twoprimary windings P1, P2 of each phase are identical to each other exceptfor their opposite connections to the DC power source. Four switchesG1-G4 are used to control current flow through the primary windings P1,P2 of each phase. The switches are used to alternately pulse DC voltagethrough the primary windings P1, P2 in order to generate steps of the ACwaveform for that particular phase from a corresponding secondarywinding S. The contributions output from the secondary windings S of thetransformers for a given phase are combined together in series toproduce the step wave AC output for that phase.

Unfortunately, this prior art configuration is bulky, requiring a threewinding, three-phase transformer 4 controlled by 12 gates for each step.Also, each primary winding P1, P2 contributes only one positive or onenegative step towards the overall AC waveform output and the totalnumber of steps of the AC output directly corresponds to the number ofprimary windings used to produce the output. To get better resolution inthis three-phase AC waveform output, therefore, more transformers mustbe added to the system, further increasing its bulkiness.

It should be noted that in each of the prior art three-phase step waveconverters, the three-phase transformers 4 used are wye-wyetransformers, meaning that both the primary P1, P2 and secondarywindings S are arranged in wye configurations. This configuration ispresumably used to avoid voltage contention which occurs between deltaand wye connections in delta-wye transformers.

A further drawback of each of the prior art power converters is that thestep wave AC output is generally blocky as a result of the mere additionof positive and/or negative block steps to form the AC waveform output.Although blocky AC waveforms are acceptable for many applications, theyare less than desirable for use in many modem electronic devices such ascomputers, televisions, etc., which perform better and last longer whenpower is supplied to them using a closely regulated AC power supply.

Therefore, the industry faces several problems related to conventionalstep wave power conversion. First of all, the industry has been unableto seamlessly integrate power from multiple power sources based on theirperformance characteristics. The industry has also failed to produce astep wave AC output that more closely approximates an ideal AC waveform.Additionally, the industry has been unable to produce a three-phase stepwave AC power output in a more efficient manner. The industry hasfurther failed to enhance the resolution of the AC waveform output froma three-phase step wave power converter without increasing the number ofprimary transformer windings. Furthermore, the industry has notsucceeded in allowing a single power source to selectively supply powerto multiple transformers when other power sources become disabled or gooffline. Nor has the industry succeeded in preventing backfeed to thepower grid or in allowing any DC power source connected to the converterto be charged from any of the other power sources connected thereto.

Accordingly, the industry would be benefitted by a step wave powerconversion method and apparatus which provides seamless integrationbetween multiple power sources. The industry would be further benefittedby a step wave AC output which more closely approximates an ideal ACwaveform. The industry is in further need of a more efficient step wavepower converter. The industry would also be benefitted by a method ofconverting DC voltage into three-phase power output with enhancedresolution with simpler circuitry. The industry is in still further needof a step wave power converter which allows a single power source toselectively supply power to multiple transformers when other powersources become disabled. Still further needs in the industry includepreventing backfeed to the power grid and allowing any DC power sourcewith storage capability connected to the converter to be charged fromany of the other power sources connected to the converter.

SUMMARY OF THE INVENTION

According to the needs of the industry, one object of the presentinvention is to seamlessly integrate power from multiple power sourcesbased on their performance characteristics.

Another object of the present invention is to produce a step wave ACoutput that more closely approximates an ideal AC waveform.

Another object of the present invention is to produce three-phase ACpower output in a more efficient manner.

Yet a further object of the present invention is to enhance theresolution of the step wave power output from a three-phase step wavepower converter without increasing the number of transformer components.

Still another object of the present invention is to selectively allow asingle power source to supply power to multiple transformers when one ormore other power sources become disabled.

Further objects of the present invention include preventing backfeedfrom the DC power buses to the input power grid and allowing any of theDC power sources connected to the converter to be charged from any ofthe other power sources connected to the converter.

This invention provides a significant improvement in the art by enablingan improved step wave power converter for converting DC voltage inputinto a step wave AC output. The step wave power converter of thisinvention is provided with multiple transformers configured to receiveDC voltage from a plurality of power sources. Each of the transformersincludes a primary winding and a secondary winding. The transformers areeach configured to supply a step for a step wave AC output. Bridgecircuits are supplied for controlling input of DC voltage into theprimary windings of the transformers. Steps for the step wave AC outputare output from the transformer secondary windings based upon the inputprovided to the primary windings. Source management circuitry manageswhich power source(s) supplies DC voltage to each of the bridgecircuits. The management circuitry provides seamless power switchingbetween the plurality of power sources based on each power source'sperformance characteristics. The step wave AC output can be a single- ormulti-phase AC output. A pulse-width modulator can also be provided tothe step wave power converter to modulate the input into a selectedprimary winding. In this way, the step wave AC output can be fine-tunedin substantial conformance with an ideal AC waveform.

A three-phase step wave power converter according to one embodiment ofthis invention includes multiple three-phase transformers. Eachthree-phase transformer has primary and secondary windings. Thethree-phase transformers are configured to receive DC voltage from oneor more power sources into their primary windings and to supply one ormore steps for each phase of a three-phase step wave AC output fromtheir secondary windings. A plurality of bridge configurations orcircuits are also supplied, each of which is made up of multiple gatepairs arranged in parallel. Each gate pair includes two or more gatesarranged in series. Opposite ends of each of the primary windings ofeach transformer are connected between gates in separate gate pairs of acorresponding bridge. Each bridge circuit is thereby configured tocontrol current flow across the primary windings of its correspondingtransformer. Preferably, the transformers are arranged having adelta-wye configuration in which primary windings are coupled in a deltaarrangement and secondary windings are arranged in a wye configuration.When configured in this way, the resolution of the three-phase step waveAC output can be enhanced by managing characteristics of the voltagetransformation between the delta primary winding configurations and thewye secondary winding configurations.

A method for enhancing a three-phase step wave AC output from athree-phase step wave power converter is also provided. The three-phasestep wave power converter has multiple three-phase transformers havingprimary and secondary windings, with each transformer arranged in adelta-wye configuration. The method begins by receiving one or more DCvoltage inputs into the step wave power converter. Steps of thethree-phase step wave AC output are generated from the secondarywindings by controlling timing and sequencing of the DC voltage inputsinto the primary windings. Voltage phase characteristics of thedelta-wye transformation are managed to increase the number of steps inthe three-phase step wave AC output.

Yet another embodiment of the invention provides a step wave powerconverter similar to those previously described, but which also includescross-tie circuitry to allow one of the power sources to supply power totwo or more transformers when one or more of the other power sourcesbecomes unstable, inoperative, or goes offline. This cross-tie circuitryincludes gated connections between two or more of the DC buses. Eachpower source can further be provided with cut-off gates to allow it tobe readily disconnected from the input system(s).

A still further embodiment of a step wave power converter includes anisolation switch for isolating at least one of the power sources fromthe input power grid to prevent or gate backfeed to the grid. It shouldalso be noted that isolation switches can be provided for each of thepower sources, to isolate each of them from each of the other powersources as well as from the input power grid. When each of the powersources are isolated from each other, bi-directional circuitry canfurther be provided for allowing any of the DC power sources to becharged from any other of the power sources. Providing isolated powersources also allows DC power to be supplied by a rectified variablefrequency and voltage input.

Finally, a method for enhancing the characteristics of a step wave ACoutput from a step wave power converter is provided in which a DCvoltage is supplied to the step wave power converter. The DC voltage istransformed into a plurality of steps of the step wave AC output.Significantly, the DC input voltage is pulse-width modulated such thatthe step wave AC output more closely approximates an AC waveform. Thismethod works particularly well when DC input voltages are provided tomultiple transformers and when the input voltage to a selected one ormore of the transformers is pulse-width modulated while holding theinputs to one or more of the other transformers in a constant on or offstate. This allows fine-tuning of the step wave AC output in substantialconformity with an ideal AC waveform.

It will be readily apparent to those of skill in the art that the abovedescribed features and advantages can be combined in numerous ways notlimited to those combinations explicitly described herein. Furthermore,the foregoing and other objects, features, and advantages of theinvention will become more readily apparent from the following detaileddescription of preferred embodiments of the invention which proceed withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a conventional single-phase stepwave power converter for converting DC voltage from a single powersource into a step wave AC output.

FIG. 2 is a schematic illustration of a conventional three-phase stepwave power converter for converting DC voltage from a single powersource into a three-phase step wave AC output.

FIG. 3 is a schematic diagram of a single-phase step wave powerconverter configured to receive and manage DC voltage inputs frommultiple power sources according to one embodiment of the presentinvention.

FIG. 4 is a series of graphs illustrating the generation of asingle-phase step wave AC output from a step wave power converter, suchas the one illustrated in FIG. 3, having four transformers.

FIG. 5A is a schematic illustration of a step wave power converter,similar to the one illustrated in FIG. 3, further including cross-tiecircuitry and cut-off gates for selectively providing or disabling inputfrom one of the DC power sources to one or more transformers accordingto another embodiment of the present invention.

FIG. 5B is a schematic illustration of a step wave power converter,similar to the one illustrated in FIG. 3, for controlling power inputsfrom multiple sources according to yet another embodiment of theinvention.

FIG. 6 is a schematic illustration of a three-phase step wave powerconverter according to yet another embodiment of the present invention,showing an improved bridge arrangement and delta-wye transformerconfigurations.

FIG. 7A is a more detailed schematic illustration of the three-phasestep wave power converter according to FIG. 6, further showing insulatedgate bipolar transistor (IGBT) modules containing bridge circuitry,driver boards for driving the bridge circuitry, control boards forcontrolling the driver boards, and series connections between secondarywindings of the transformers for each phase, among other things.

FIG. 7B is an enlarged view of the transformer configuration of theconverter in FIG. 7A.

FIG. 7C is a block diagram of a control board having both software andhardware components for controlling the step wave power converter ofFIG. 7A according to a preferred embodiment of the present invention.

FIG. 8 is a voltage versus time graph showing a step wave AC output froma delta-wye three-phase step wave power converter similar to FIG. 8A buthaving enhanced resolution resulting from careful control of voltagecharacteristics in the delta-wye transformers.

FIG. 9 is a flow chart illustrating a method for more accuratelyapproximating an ideal AC waveform using a hybrid of step wave powerconversion and pulse-width modulation according another embodiment ofthe invention.

FIG. 10 is a voltage ratio versus time graph illustrating operation ofthe hybrid step wave and pulse-width modulation power conversion methodof FIG. 9.

FIG. 11A is a schematic illustration of a prior art uninterruptiblepower supply system for providing backup power.

FIG. 11B is a schematic illustration of the step wave power converter ofthe present invention for use as a backup power system according to yetanother embodiment of the invention.

DETAILED DESCRIPTION

The step wave power converter (SWPC) of this invention is an innovativepower converter designed around a unique platform that allows it to havea wide range of uses beyond those of conventional power converters.These uses extend beyond the usual task of converting power from asingle DC source to AC power. One such use includes consolidation,integration and supervisory control of multiple power sources through asingle SWPC while isolating each source so that each can operate atoptimum efficiency. The power sources connected to the SWPC can includediesel or gas generators, wind turbines, solar photovoltaic (PV) cellarrays, hydroelectric generators, batteries, gas turbine generators,fuel cells, etc. Yet another use is in backup power supply systems,including integration, isolation, and management of the power sourcesthat comprise the backup power supply system. Still another use ismanaging the power for power generators installed in the distributedgeneration mode. Another use is end of grid and in line voltage andpower quality regulation. Further uses include standard 60 Hz orcustomized frequency regulation; the ability to feed reactive power to agrid or an off-grid load on demand; and the provision of a programmablemicroprocessor controller that is customized and optimized, as required,for each application.

Specific embodiments of the present invention will now be described inmore detail. FIG. 3 is a schematic illustration of a single-phase stepwave power converter for receiving and managing DC voltage inputs frommultiple power sources according to one embodiment of the presentinvention. According to this embodiment, DC buses 5 receive power fromthe power sources and supply it as a DC voltage input to one or morebridge circuits 10. Each bridge circuit 10 preferably consists of aninsulated gate bipolar transistor (IGBT) module having four IGBTswitching gates G1-G4, which are controlled by a driver board inresponse to signals from a control board. Each IGBT switching gate G1-G4is preferably fitted with an antiparallel diode D1-D4, respectively, toallow shorting current to flow. Although IGBT switching gates arepreferred, the gates can include HEXfets or other semiconductor powerswitching devices and a corresponding antiparallel diode. In thisembodiment, a single two winding (one primary P and one secondary S)transformer 15 is used for each step.

Single-phase shorting using the four gate bridge 10 involves closing thetwo gates G1, G2 on the positive inputs (the positive transistors) orthe two gates G3, G4 on the negative inputs (the negative transistors).Closing the gates in this manner allows shorting current to flow throughone diode and one gate of a shorted transformer 15, thereby imposing anull potential across the primary winding P of the shorted transformer.Shorting is important for allowing power supplies to be dynamicallyadded or removed from a transformer without affecting the transformer'swinding ratio requirements.

FIG. 4 illustrates the production of a single-phase step wave AC outputfrom a step wave power converter, such as the one described above withreference to FIG. 3. Referring now to FIG. 4, a step wave AC output isproduced as follows. In a step wave power converter having fourtransformers, each transformer produces an output from its secondarywinding according to a voltage input into its primary winding and thetransformer winding ratio. Each of these outputs forms a building block,or step, of an overall AC output. The outputs from all of the secondarytransformer windings are added together in series to simulate the ACsine wave.

Generally, the process for producing the step wave output proceeds byturning on each of the transformers sequentially at a specified time andthen leaving them on for a given period of time before sequentiallydeactivating them. Specifically, this process begins by turning on afirst transformer at a zero reference time t0. The activation of thefirst transformer activates step one of the step wave output. Step oneremains activated while other steps are added. At a first point in timet1, a second transformer is turned on and its voltage output is combinedwith the output of the first transformer, thereby activating step two.Similarly, at a second point in time t2, a third transformer is turnedon and its voltage output is added to that of the others to activatestep three of the step wave output. Likewise also, at a third point intime t3, a fourth transformer is turned on to activate step four.

At a later specified time, the step wave production process is reversedto step the AC waveform back down. This is accomplished by sequentiallyturning the transformers off at fourth, fifth, sixth, and seventh pointsin time t4, t5, t6, and t7. Turning a transformer off preferablyincludes shorting a voltage across the primary winding of thetransformer as described above. Although this reverse process canproceed by turning off the transformers in any order, a preferred methodproceeds by deactivating the transformers in the order they wereactivated. Accordingly, the first transformer is deactivated first, thesecond transformer next, and so on. Specifically, step one isdeactivated by turning off the first transformer at the fourth point intime t4. Step two is deactivated at the fifth point in time t5 byturning off the second transformer. Similarly, step three is deactivatedat the sixth point in time t6 by turning off the third transformer. And,finally, step four is deactivated at the seventh point in time t7 byturning off the fourth transformer. By deactivating the transformers inthe order they were activated, balancing of the transformer duty cyclesis achieved.

Although not shown, after all of the transformers have been turned off,the waveform building process is repeated in order to build the later180 degrees (or negative half) of the AC sine wave. The process forproducing the negative half of the waveform is the same as for thepositive half just described, except with negative voltage polarity.

Referring again to FIG. 3, a positive step for the AC waveform isgenerated by closing the first positive switch G1 and the secondnegative switch G4 in one of the bridge circuits. A negative step forthe AC waveform is generated by closing the first negative switch G3 andthe second positive switch G2 in one of the bridge circuits. A shunt ofthe transformer primary P is generated by closing either both positiveswitches G1, G2 or both negative switches G3, G4.

In summary, the steps of the simulated AC waveform of FIG. 4 areproduced by sequentially enabling and disabling DC voltage input intothe primary windings of multiple transformers at specified points oftime. In some embodiments, each step may be formed from the voltagecontributions of only one transformer. In other embodiments, however,each step may be formed from the voltage contributions of more than onetransformer.

Voltage control of the step wave AC output is established by varying thenumber of transformers active at any given time as well as the dutycycle associated with each of those transformers. The transformers canalso be sized to assure that any number less than the total number oftransformers are capable of producing rated output voltage.Additionally, by adding steps and by varying the duty cycle of any givenstep, a wide range of output voltages can be derived. Additionally, stepwidth can be varied to generate the proper waveform and RMS voltage.

When multiple power sources are provided to a step wave power converter,it is sometimes desirable to maintain the ability to cross connect anyof the power sources to any number of transformers in the converter.Consequently, according to a preferred embodiment of this invention,each power source connected to the SWPC is supplied with a bypassswitch. Bypass switches allow the SWPC to switch off an abnormal powersource. Bypass switches further allow the SWPC to prevent backfeed tothe grid. Bypass switches can be added to any SWPC configuration bysupplying cut-off gates to the power source input lines as illustratedin FIG. 5A. This allows the gating mechanism of the grid source to beblocked when needed. Another layer of protection can be achieved usingthe cross-tie approach described below.

FIG. 5A shows additional DC source management circuitry including across-tie arrangement for interconnecting multiple power sources withmultiple transformers. In the cross-tie arrangement, a step wave powerconverter is provided with gated interconnections, called cross-ties,between DC buses 5. The gates on the interconnections are referred to ascross-tie gates. The cut-off gates described above are included on eachpower source's positive and negative input lines to isolate that powersource from the other power sources and the grid. In normal operation,the power source cut-off gates are closed to allow power to be suppliedfrom each of the power sources while the cross-tie gates are open toprovide isolation between DC buses. When one of the power sources failsor is disconnected, however, a degrading DC bus 5 is sensed. The cut-offgates associated with the failed source then open to isolate and preventfurther contribution of power from the compromised power source, and thecross-tie gates close to allow a still functioning power source tosupply power to the DC bus 5 for the compromised power source. Thiscontrol mode assures seamless transfer between power sources while stillmaintaining isolation between them. Although FIG. 5A shows only twopower sources, it should be appreciated that this embodiment is scalableto include any number of power sources and cross-tie devices. Therefore,more than two sources can be added in this scheme.

An improvement in the art realized by yet another embodiment of thisinvention results from the provision of bi-directional circuitry betweenthe isolated power sources. Bi-directional circuitry between isolatedpower sources gives the SWPC of this invention the ability to charge anyof the DC sources connected to the SWPC from any of the other sourcesconnected to the SWPC. In other words, this circuitry enables abi-directional capability on any of the DC buses 5 but maintains theirisolation from one another. For example, in a SPWC where a battery and aphotovoltaic (PV) cell array comprise two of the power sources, thebattery can be charged from the PV array while still maintaining thearray's isolation from the battery. This is a significant innovationbecause the batteries can stay at a relatively constant voltage whilethe PV maximum power point voltage fluctuates.

A still further advantage provided by the use of isolated DC buses isthe ability of the SWPC of this invention to allow variable speedoperation of any combination of rotating or fixed power generationmeans. For example, a variable speed diesel, a variable speed windmill,and a PV array can all be run through a single SWPC when isolation ismaintained between each of the DC buses. In other words, when thediesel's rectified DC bus is isolated from the rectified DC bus of thewindmill and the PV array, each of the sources can operate at anydesired speed or voltage level without interfering with the othersources.

FIG. 5B illustrates still other embodiments of this invention which arealso configured to provide multiple power source management throughpower source management circuitry. One such embodiment includes multiplehigh frequency input converters tied to each DC bus 5, while yet anotherincludes a combined multiple input converter. By using more than onehigh frequency input converter tied to each DC bus 5, or a combinedmultiple input converter, each input may contribute as much of the powerto the overall system as desired. The top circuit illustrated in FIG. 5Bshows multiple high frequency input converters tied to each DC bus 5. Inthis embodiment, power inputs from each of the multiple power sourcesare run through a separate isolation circuit which can also containpulse width modulation circuitry. One of the input power sources, i.e.,Input #1, can be an input power grid. The outputs from all of theisolation circuits are combined together and supplied to the DC bus 5.Each DC bus 5 can then be used to supply power to a transformer. Thetransformer would receive DC voltage input from the DC bus, whichreceives power from one or more power sources, including the input powergrid, through the isolation circuit. The isolation circuit can therebyisolate the DC bus from the input power grid to prevent backfeed to thegrid from the DC bus.

The circuit illustrated at the bottom of FIG. 5B illustrates a combinedmultiple input converter tied to a DC bus 5. In this converter, multiplehigh frequency DC/DC, PFC, and AC/DC converter inputs from multiplesources may be converted to a common DC bus 5. By providing properfeedback control, each input can supply a regulated portion of the powerused in the inverter. The portion of the power supplied by each inputcan be adjusted by the control board. This feature can also beincorporated into a single, high frequency converter circuit, withmultiple inputs, that synchronizes control and reduces components.

Multiple power source management is particularly beneficial where someor all of the power sources produce non-uniform power outputs, such asphotovoltaic cells, windmills, etc. According to this invention, suchsources could be used to provide a large amount of the power when theirstrength is high, but be used to supply less of the power as theyweaken. The control signal for each input converter will determine theamount of power transferred from each power source. This embodimentthereby facilitates “soft” transfers between input sources. Unlike“hard” power transfers, where a power source is either connected to ordisconnected from the system, “soft” power transfers allow each powersource to contribute a desired percentage of the power to each of thetransformers. Also, this invention allows power sources to be slowlyramped-in or ramped-out when being connected to or disconnected from thesystem, helping to prevent voltage spikes and provide a more uniformpower supply. These types of multiple power source control can beutilized with either single-phase or three-phase power converters.

Still another embodiment of this invention provides improvements in theart specifically with respect to three-phase step wave power converters.This embodiment is the unique SWPC configuration shown in FIG. 6. FIG. 6is a schematic illustration of a three-phase step wave power converterincluding an improved bridge architecture 20 and delta-wye transformers25. Specifically, this embodiment utilizes a unique step wave powerconverter topology consisting of multiple three-phase transformers 25,each arranged with primary windings PA, PB, PC in a delta configurationand secondary windings SA, SB, SC in a wye configuration. Voltage flowacross the primary windings PA, PB, PC of each transformer is controlledby six gates G1-G6 configured in a bridge circuit 20. One or more powersources can be used to supply power to the bridge circuits 20 throughtheir respective DC buses 5. Each of the gates G1-G6 in a bridge circuit20 includes an insulated gate bipolar transistor (IGBT) fitted with anantiparallel diode to allow shorting current to flow. A primary benefitof this new topology is that it requires only six gates G1-G6 and onethree-phase transformer 25 per step (having only one primary and onesecondary winding per phase) rather than the nine or twelve gates andthe more complex transformer configurations of the prior art.

As mentioned above, the connections between the primary windings of eachof the three-phase transformers 25 in this embodiment are arranged in adelta configuration. Each three-phase transformer 25 includes a singleprimary winding PA, PB, or PC and a single secondary winding SA, SB, orSC for each phase. In the delta configuration, a first end of a phase Aprimary winding PA and a second end of a phase C primary winding PC ofone transformer 25 are coupled together and connected to thattransformer's bridge circuit 20 between two gates G1 and G4 in a firstseries connected gate pair. Similarly, a second end of the phase Aprimary winding PA and a first end of the phase B primary winding PB arecoupled together and connected to the bridge circuit 20 between twogates G2 and G5 in a second series connected gate pair. Finally, a firstend of the C phase primary winding PC and a second end of a B phaseprimary winding PB are coupled together and connected to the bridgecircuit 20 between two gates G3 and G6 in a third series connected gatepair. The secondary windings SA, SB, SC of each three-phase transformer25 are arranged in a wye configuration, with all of the secondarywindings SA, SB, or SC of the same phase being connected together inseries.

Operation of the three-phase transformers 25 using the six gate bridge20 will now be described in more detail. Voltage across the primarywindings PA, PB, PC of the transformers is controlled to induce thesteps of the AC waveform output for each phase A, B, C through thecorresponding secondary windings SA, SB, SC. Each of the transformers 25directly contributes one step to the AC output of each phase.Specifically, when a voltage is applied across a transformer's primarywinding corresponding to one of the phases, the corresponding secondarywinding produces a step for that phase of the AC output. Furthermore,similar to the single-phase embodiment, a voltage is shorted across oneor more of the primary windings of the three-phase transformer 25 inorder to shunt them. Three-phase shorting (i.e., shorting of all threephases) using a six gate bridge 20 involves closing the three positivetransistors G1-G3 or the three negative transistors G4-G6. Closingeither set of three gates allows shorting current to flow through acombination of diodes and gates so that a null potential is imposedacross all three primary windings PA, PB, PC of the shorted transformer.

Furthermore, each of the primary windings PA, PB, PC of the transformer25 may have a potential or be shunted at different times based upon theoperation of the six gates G1-G6. For instance, the phase A primarywinding PA will be on when either of two sets of gates G1, G5 or G2, G4are closed. Positive polarity voltage is applied across the phase Aprimary winding PA when a first positive gate G1 and a second negativegate G5 are closed. Conversely, reverse polarity voltage is applied tothe phase A primary winding PA when a second positive gate G2 and afirst negative gate G4 are closed. Phase A is shorted and turned off,however, when either the two positive gates G1, G2 or the two negativegates G4, G5 connected to opposite ends of the phase A primary windingPA are closed. Similarly, the phase B primary winding PB will be on wheneither of two sets of gates G2, G6 or G3, G5 are on. Positive polarityvoltage is applied across the phase B primary winding PB when the secondpositive gate G2 and a third negative gate G6 are closed. Reversepolarity voltage is applied across the phase B primary winding PB when athird positive gate G3 and the second negative gate G5 are closed. Thephase B winding PB is turned off when either the two positive gates G2,G3 or the two negative gates G5, G6 connected to its opposite ends areclosed. Phase C is again similar. Positive polarity voltage is appliedacross the phase C primary winding PC when the third positive gate G3and the first negative gate G4 are both closed. Reverse polarity voltageis applied across the phase C primary winding PC when the first positivegate G1 and the third negative gate G6 are closed. Finally, the phase Cprimary winding PC is turned off when either the two positive gates G1,G3 or the two negative gates G4, G6 connected to its ends are closed.

It should be appreciated that the gates G1-G6 may be controlled in anynumber of combinations in order to produce the desired steps for eachphase. Accordingly, by controlling the six gates G1-G6 of the bridgecircuit 20, voltage of either positive or negative polarity or a nullpotential can be applied across the primary windings for each phase. Inthis way, the desired contribution to the overall AC waveform can beoutput from the phase's corresponding secondary winding based on controlof the bridge circuit 20.

FIG. 7A is a detailed schematic illustration of a three-phase step wavepower converter (SWPC), such as the one described above with referenceto FIG. 6. FIG. 7B is an enlarged view of the transformer arrangement ofthe SWPC of FIG. 7A. Referring to FIG. 7A, IGBT modules provide thebridging circuitry 20 for control of DC power into the primarytransformer windings of three-phase transformers. Power is supplied froma power source to DC buses 5. The DC buses 5 supply DC voltage input toterminals N and P of each IGBT module 20, where terminal N is the DCnegative terminal and terminal P is the DC positive terminal. Each ofthe IGBT modules 20 produces three separate outputs A, B, and C from itsthree output terminals U, V, and W. These outputs are the buildingblocks for the A, B, and C phases of the three-phase AC output.

In this embodiment, four IGBT modules 20 are used to control when DCvoltage inputs are supplied to the primary transformer windings of fourthree-phase transformers 25 to produce the steps (or building blocks)for each of the three phases. Of course, more or fewer than four IGBTmodules 20 and transformers 25 could be used. The ratio between IGBTmodules 20 and transformers 25 is typically one to one. Each three-phasetransformer 25 includes three primary windings and three secondarywindings (one of each for each phase). Also, in this embodiment, eachIGBT module 20 is supplied power from a single, separate DC power bus 5,each of which is connected to its own power source (Power Sources 1-4).It should be appreciated, however, that any number of power sources maybe connected to any one or more of the DC buses 5, as has been describedabove with respect to other embodiments of the invention.

The IGBT modules 20 regulate the flow of current from their DC bus 5across the primary windings of their corresponding transformers 25 inorder to produce the steps of the three-phase AC waveform. The four IGBTmodules 20 are each controlled by one of four driver boards 22 that are,in turn, controlled by a control board 24. More specifically, a controlalgorithm, resident on the control board 24, controls signals sent toeach of the four driver boards 22 that, in turn, send signals toactivate the gates inside of each of the four IGBT modules 20 at theappropriate times. The control algorithm thereby controls the activationof the IGBTs in a desired sequence to produce the step wave AC output.

Referring now to FIG. 7B, outputs A, B, and C from the IGBT modules 20are fed to the primary windings PA, PB, PC of their correspondingthree-phase transformers T1-T4 to control voltage therein. Eachtransformer T1-T4 directly produces a single step for each phase fromits secondary windings SA, SB, SC based on current flowing through itscorresponding primary windings PA, PB, PC. The four transformers T1-T4are configured having secondary windings of the same phase connectedtogether in series. The three phases are also connected into a wyeconfiguration on each of the secondaries.

As mentioned above, the three-phase step wave power converter of thisembodiment has control circuitry including three types of controldevices, as shown in FIG. 7A. The control board 24 has all programmedinformation and is the heart of the control system. The driver boards 22are an interface between the control board 24 and the IGBT modules 20.The IGBT modules 20 are the power electronics that allow the electricalside of the step wave power converter to operate. The IGBT modules 20are preferably commercially available Powerex six pack modules made byPowerex Intellimod and are unmodified from their original condition. Thedriver boards 22 are generally known to those skilled in the art. Thecontrol board 24 is being designed and built specifically for use withthe single- and three-phase step wave power converters of thisinvention, a schematic representation of which is shown in FIG. 7C.Resident on the control board 24 is the micro-controller chip that isused to control all aspects of the step wave power converter. Thesoftware in the control board 24 enables the unique switching aspects ofthe invention.

The control board software manages the operation of the entire SWPC. Itcontrols the operation of all of the IGBT switches within each of theIGBT modules 20 that in turn characterize the AC waveform. The propertiming for operating each of the switches in each IGBT module 20 iscrucial to generating acceptable AC power quality. The software alsoprovides features such as the ability to maintain loading for individualinput sources; control of AC output voltage and current; phasing andgrid synchronization; the ability to monitor and isolate gates and orgate driver logic failures; the ability to skew step wave timing toreduce harmonic distortion; step wave/pulse-width modulation (PWM)hybrid control; the ability to combine multiple inputs with differentvoltages and ratings; the ability to provide feedback to power sourcesto allow following of output loading; and the ability to allow heavyloading of single inputs, such as batteries, for a short period of timeduring transient conditions to allow for sources with a slower reactiontime to pick up loads. Although the foregoing and other features arepreferably implemented by software, it should be noted that some or allof these aspects of the invention may be performed in analog circuitryrather than by software.

In a basic three-phase step wave power converter, each phase of the ACwaveform can be constructed in the same way as was described previouslywith respect to the single-phase step wave AC output shown in FIG. 4.Although the basic stepping procedure works very well with thesingle-phase power converter, however, using it directly on thethree-phase power converter of this invention produces contentionbecause of the delta-wye configurations of the gates and transformers.This contention is very detrimental to the AC power output quality.Remarkably, however, in the three-phase delta-wye configuration, thetiming of the IGBT switches can actually be controlled in such a way(i.e., to adjust phase shift instead of step width) that the transformerphase shift is used in a constructive, rather than a destructive,manner. A phase management controller, such as the control board 24, maybe used to control the switches to use transformer phase shiftconstructively.

Therefore, although the arrangements of the primary and secondarywindings of three-phase transformers can be configured many differentways, it becomes advantageous in this invention to configure the primarywindings in a delta arrangement and the secondary windings in a wyearrangement. Specifically, this delta-wye arrangement, when properlycontrolled, allows the resulting step wave to be made to contain n+2steps, where n is the number of transformers involved in producing theAC waveform. The AC waveform thus produced includes additional stepscaused by the addition of three phase-shifted primary waveforms.

FIG. 8 is a voltage versus time graph showing an enhanced step wave ACoutput produced by carefully controlling the IGBT switches to use phaseshift between delta and wye transformer winding configurationsconstructively. As shown, constructive use of transformer contention canprovide a six step waveform using only four transformers, rather thanthe conventional four step waveform.

To obtain the improved results described above, the invention provides aunique method for controlling the IGBT switches that allows thethree-phase step wave power converter to produce an AC output voltagewith higher resolution. This higher resolution includes an increasednumber of output voltage steps while using the same number of IGBTswitches and transformers. This unique control method combines thenormal phase shift combinations in the delta-wye transformerconfiguration with smart transformer phase shift control logic to reduceharmonics. In other words, by intelligently activating and deactivatingthe switches on the IGBT boards according to the natural delta-wye phaseresponse, the step wave power converter of this embodiment provides anenhanced step wave AC output signal.

As discussed previously, use of conventional step wave switchingalgorithms to produce a simulated AC output is well-known. It is alsoquite common for conventional inverters to utilize a pulse widthmodulation (PWM) switching algorithm to approximate a sine wave. PWMrefers to the change of the on and off times (duty cycle) of pulses,such that the average voltage is the peak voltage times the duty factor.In such PWM inverters, a sine wave is approximated using a series ofvariable-width pulses. None of the prior art power converters, however,have combined a step wave output with PWM. A significant improvement inthe art is provided by this invention through a novel combination ofstep wave power conversion and PWM.

Fortunately, both step wave and PWM processes can cycle power sources inany sequence as well as control individual input source loading. Severaladvantages therefore result from the combination of these twoapproaches. These advantages include, among other things, closerapproximation to a sine wave than with either of the prior artapproaches alone, fewer losses than in conventional pulse-widthmodulation approaches, elimination of the need for rapid switching offull line voltage, and greater adaptability of the AC waveform output.

Accordingly, still another preferred embodiment of this inventionutilizes a unique combination of step wave and PWM algorithms togenerate a hybrid step wave/PWM AC output that very closely approximatesan ideal AC sine wave (i.e., potentially less than 2% total harmonicdistortion). FIG. 9 is a flow diagram of a preferred algorithm forcombining PWM with step wave power conversion. This flow chartillustrates a process for creating a hybrid step wave/PWM waveform thatclosely approximates an ideal AC waveform. It should be noted that thisalgorithm can be incorporated as firmware on a micro-controller withsupporting analog circuitry or it can be completely analog or completelymicro-controller based.

Generally, according to this novel approach, PWM is used to improve thetransition edges of each step of a step wave AC output. The hybrid stepwave/PWM system uses a pulse-width modulator to modulate the power inputinto a selected one of the transformers while inputs into the othertransformers are held in a steady on or off position, to maintain thebasic steps of the AC step waveform. PWM waves are thereby used in thestep wave transitions to refine the envelope of the simulated ACwaveform. These smaller PWM pulses can be filtered to help produce awell regulated sine wave that has very little harmonic distortion. Inthis way, the step wave process is used to approximate an AC sine waveon a large scale while the PWM process provides higher refinement to thesine wave approximation. The combination of using PWM for one or moretransformers while using step wave power conversion technology forothers is unique.

FIG. 10 is a graph that further illustrates the hybrid process forcreating an AC waveform as described above using the algorithm of FIG.9. The vertical axis of the graph represents the ratio of the totalvoltage output V(out) from series combined secondary transformerwindings of the SWPC to a peak voltage Vsetpoint of an ideal sine wave.The horizontal axis is a time axis. The bottom graph represents the PWMoutput supplied to a selected primary winding of one of thetransformers. Generally, as illustrated by the graph of FIG. 10, thehybrid step wave/PWM approach works by adding the step waves together togenerate a rough estimate of a sine wave while pulse-width modulatingvoltage input signals during transitions to smooth the edges of thesteps.

Referring to FIGS. 9 and 10, the hybrid step wave/PWM algorithm will nowbe described in detail. First, however, the parameters of the algorithmneed to be defined. V(i) is used to represent the voltage applied at aprimary winding i, where i=1, 2, 3, 4, . . . , k; and where k representsthe total number of transformer primary windings used in generating theAC waveform. As noted previously, V(out) represents the combined outputvoltage from the series connected secondary transformer windings andVsetpoint indicates the maximum voltage level of an ideal AC waveform.The PWM envelope represents the limits within which the PWM operationtakes place, such that the unfiltered pulses are bound by the PWMenvelope.

When the hybrid step wave/PWM process begins, the combined outputvoltage V(out) is at zero and the parameter i is set to 1. PWM of a DCinput voltage V(1) into a first primary winding of a first transformertherefore begins. Accordingly, the input voltage V(1) begins to begradually supplied to the first primary winding, such that the firsttransformer is turned on. As the voltage V(1) supplied to the firstprimary winding is modulated and filtered, it gradually increases, asshown by the PWM output graph 26 at the bottom of FIG. 10. The outputvoltage from the first transformer's secondary winding and the combinedoutput voltage V(out) increase correspondingly. This input voltage V(1)is continuously modulated as shown by signal 28 until the PWM levelreaches 100% for that step at time 30. Once PWM for that step reaches100%, the input voltage V(1) into the first primary winding iscontinuously turned on, as represented by line 32, and the parameter iis then incremented by one so that an input voltage V(2) into a secondprimary winding of a second transformer can then be modulated, asrepresented by signal 34.

The PWM process described above is repeated for the voltage inputs toeach of the primary windings until the last required primary winding kis reached. When this occurs (i.e., when i becomes equal to k), pulsewidth modulation of the input voltage V(k) into the final primarywinding begins and continues until the overall output voltage V(out)becomes equal to the maximum voltage Vsetpoint of the ideal AC waveform.When the output voltage V(out) reaches this point, it is at its maximumdesired value and must therefore begin to be decreased. To decrease theoutput voltage V(out), i is reset to 1 and the PWM process is reversed.

It should be noted that during modulation of each of the voltage steps,the combined output voltage level V(out) is continuously tested to seeif it has reached its maximum desired value Vsetpoint. As long as theoutput voltage V(out) remains below the maximum, however, PWM of thecurrent step continues until it reaches 100% for that step, as describedabove. When the voltage output level V(out) reaches its maximum desiredvalue, i is reset to 1 and the PWM process is reversed so that thevoltage can be gradually reduced, whether or not all of the primarywindings have been used.

The PWM process continues by gradually reducing PWM of input voltageV(1) to zero, as represented by signal 36. Once PWM reaches 0% for thatstep, its input voltage V(1) is turned off continuously and i isincremented by one so that the input voltage V(2) into the secondprimary winding can be modulated, as represented by signal 38. Thisprocess continues for the voltage inputs for each of the primarywindings 1-k as they are each gradually reduced to zero. After PWM ofthe final input voltage V(k) (signal 40) has been completed and thevoltage output V(out) is zero (at time 44), the parameter i is againreset to 1. The entire process then repeats, except this time withnegative polarity as shown in 42.

As a result of the hybrid step wave/PWM process described above, it isbelieved possible to create a simulated AC waveform with less than 2%total harmonic distortion. This invention therefore provides asignificant improvement in the art by enabling a SWPC which produces asimulated AC waveform which very closely approximates an ideal ACwaveform.

A few specific applications for this invention will now be describedfurther. One specific application for the use of an SWPC havingmultiple, controllable, isolated source inputting is in hybrid renewablepower systems. SWPCs of this invention can seamlessly and efficientlyintegrate renewable energy sources such as hydro, wind, and solar powerwith conventional generators such as diesel and gas turbines inoff-grid, end-of-grid, and on-grid applications without compromising theefficient operation of the conventional or the renewable power generatorunits. Using such an SWPC permits the renewable power sources to be usedas the primary sources while still ensuring continuous operation,thereby reducing fuel consumption of the conventional power generators.

Yet another use for the present invention is in backup power systems.Backup power systems are used to provide power to facilities when theutility grid fails. These systems usually consist of a diesel generator(the primary power supply when operating off-line), batteries thatprovide temporary power during generator start-up, a power inverter thatinverts the DC battery or generator output to AC power, and a staticswitch that transfers the load from the utility grid to the backup powersupply when needed. This entire system is conventionally referred to asan uninterruptible power supply (UPS). Unfortunately, most UPS systemssuffer from one significant shortcoming—if one of the major componentsfails, the entire system is compromised.

More specifically, in a typical UPS system, such as the one shown inFIG. 11A, a utility grid 50 and a backup power system (generator) 52 arenot synchronized. A transfer switch 56 selects between the two powersource input lines #1 and #2 depending on which power source 50 or 52 isdesired. When the utility grid 50 fails, the backup power line #2 isactivated to supply power from the backup power source 52. One or morebatteries 54 provide temporary DC power that is inverted to AC power forthe user. After the generator 52 comes to normal operating speed, powerwill be provided solely by the generator 52. A rectifier 57 is used torectify the power from the utility grid 50 or the generator 52 to DCpower. An inverter 58 inverts the incoming DC power to AC power. Becauseof this interdependent component configuration, if any one of thecomponents fails, the entire system is compromised.

Unlike the conventional system, the SWPC of this invention, when used ina UPS application as schematically illustrated in FIG. 11B, canaccommodate and integrate multiple power sources 50, 52, 54. The abilityto integrate multiple power sources gives the SWPC 18 importantadvantages over the typical UPS systems. First of all, the inventioneliminates the need for the transfer switch 56 (see FIG. 11A) that isused with many UPS systems. This invention therefore provides trulyseamless “uninterruptible” power. This invention also preferablyisolates each power source 50, 52, 54 from the system to providecontinuous voltage regulation. If one of the power sources, such as theutility grid 50, becomes inactive or is deliberately disconnected, thisembodiment of the invention will regulate the power output using theremaining power sources 52, 54. This feature can eliminate costly downtimes by allowing scheduled service of power supplies without affectingthe user. When combined with the rugged and reliable design of the SWPC18 relative to commercial inverters that reside within the UPS, thisarchitecture is much more reliable and useful than a typical UPS system.

The SWPC 18 can also condition the power from the utility grid that isto be used with sensitive electronics—a process that conventionallyrequires additional equipment provided by the end user. This providesimproved efficiency, regulation, and isolation over the use offerro-resonant transformers, as are conventionally used. The flexibilityof the SWPC 18 also gives the end user room for expansion ormodernization of power sources. For example, an existing dieselgenerator 52 or battery bank 54 could be replaced with fuel cells asthey become available.

Yet a further application of this invention is in integrating power fromphotovoltaic (PV) cell or battery arrays. PV cells and batteries arepower sources for which the SWPC of this invention is ideally suited.This is because these DC sources are typically made up of multiple,independent “strings”. PV arrays, for example, typically consist ofmultiple strings of PV cells. Larger battery banks are also typicallyarranged as parallel strings of batteries and will benefit from use ofthe SWPC. Each string delivers power as a DC output voltage. The SWPC 18can treat each string as an independent source and electricallyintegrate the multiple strings, while maintaining isolation betweenthem. This is a key advantage of the SWPC 18 because if one or morestrings malfunction, the SWPC 18 can continue to deliver utility- orelectronic-grade AC power from the still-functioning strings.

Additionally, the SWPC 18 of this invention can cater to various nominalDC voltage levels among the strings. Existing inverter systems deal withnominal DC voltage levels through individual voltage regulators on eachstring or by merging all of the DC power on a single DC bus and theninverting the power from the bus to AC power. In some cases, invertersare attached to each string and the AC power from each inverter iscombined to feed the load. The SWPC 18 drastically simplifies andimproves the power conversion architecture compared with prior artinverters for PV arrays allowing maximum power point tracking of eachinput.

A still further application of this invention exists with respect tofuel cells. Fuel cells create electricity using an electrochemicalprocess. They differ from batteries, which also use an electrochemicalprocess, in that they consume hydrogen and must therefore have fuelcontinuously provided. The type of fuel used to generate hydrogen variesand depends on the reforming process for which each system is designed.Fuel cells are well suited for distributed generation, but each systemmust be tailored to the application that it will serve. Someapplications may require higher power quality than others; some may needto be interconnected with the utility grid; some may require severalfuel cells to be paralleled together; while some may implementco-generation where waste heat of the fuel cell is used along with theelectrical energy. All these applications require power conditioning andcustom electrical interconnections with the end user's facility.

The electricity generated from fuel cells is also DC and must beregulated or converted to AC for user consumption. Conventionally, thisis accomplished by using a power converter that is often not integratedinto the design. The SWPC 18 of this invention offers clear advantagesover present techniques. One primary advantage in fuel cell integrationoffered by the SWPC 18 is parallel operation of multiple fuel cellswhere each unit may be individually loaded. Another advantage is theability to follow fuel cell voltage versus loading curves and limits.

Two conventional methods exist for consolidating the power of multiplefuel cells servicing a single user. One method is to use voltageregulators for each fuel cell and to have a common bus to which thesevoltage regulators feed power. The power from the common bus is thenconverted by a single power converter and fed to the user. A secondmethod is to use a power converter for each fuel cell, combine theconverted power and feed it to the user. Both of these techniques arecostly because of the duplication of system components for each fuelcell.

Referring again to FIGS. 11A and 11B, the SWPC 18 has a distinctadvantage over the prior art described above. Specifically, one or morepower sources, i.e., 52, 54, for the SWPC 18 can be a fuel cell. Byreplacing the battery banks 54 with fuel cells, the SWPC 18 allows eachfuel cell 54 to operate at peak efficiency by isolating each fuel cell54 from the others, as with the other power sources already describedabove. The SWPC 18 converts the power to AC and supplies it to the user.This simplifies the architecture and allows one or more of the fuelcells 54 to be taken off-line without any adverse effects.

Still further benefits of this invention exist with respect togrid-connected applications that have one or more inputs from the grid.In present input grid-connected applications, the fuel cell 54 isconnected to an inverter in synchronization with the utility grid 50. Itis disconnected from the grid 50 (i.e., for servicing) using a transferswitch 56. The SWPC 18 of this invention offers a clear advantage overthe transfer switch 56. Both the utility grid 50 and the fuel cell 54,or multiple fuel cells 54, are used as power sources for the SWPC 18.The SWPC 18 conditions the power for the user and isolates each fuelcell 54 from the utility grid. The SWPC 18 allows each fuel cell 54 tooperate under the preferred conditions for fuel efficiency orco-generation. In addition, since all power sources are isolated by theSWPC 18, there is no need for a costly transfer switch 56 in the eventone of the sources fails. The SWPC 18 will simply use the remainingpower sources 50, 52, 54 to create high quality power.

Fuel cells 54 can also provide power when the grid 50 is not active orwhen there is no grid available. When power from the utility grid 50 islost, the fuel cell 54 will provide backup emergency power for the user.For UPS systems, the fuel cell 54 can effectively replace the dieselgenerator source 52, which is commonplace today.

The invention's ability to integrate multiple power sources furthergives the end user the ability to expand the system power capacity inthe future without costly system upgrades or the purchase of an entirelynew system. With simple software modifications, the SWPC 18 of thisinvention can be upgraded to accommodate multiple fuel cells,interconnection with the utility grid, or to parallel other types ofpower sources with the fuel cell.

Having described and illustrated the principles of the invention inseveral preferred embodiments thereof, it should be apparent that theinvention can be modified in arrangement and detail without departingfrom such principles. Particularly, the features and advantages of allof the various embodiments can be arranged together in any combination,depending only on the desired application. We therefore claim allmodifications and variations coming within the spirit and scope of thefollowing claims.

We claim:
 1. A step wave power converter comprising: a plurality oftransformers, each configured to selectively receive a DC voltage fromone or more of a plurality of independently-generated power sources,including at least one variable power source, each transformer beingconfigured to selectively supply one or more steps for a step wave ACoutput; a plurality of bridge circuits, each adapted to selectivelysupply DC voltage input from one or more of the multiple independentlygenerated power sources, including the variable power source, into oneor more of the transformers to generate steps for the step wave ACoutput from the transformers; and source management circuitry capable ofevaluating performance characteristics of each power source, includingthe variable power source, and managing how and when each of theindividual DC voltage inputs is supplied by the bridge circuits to thetransformers based on those performance characteristics.
 2. The stepwave power converter according to claim 1, wherein the source managementcircuitry determines an amount of power available from each of the powersources at a particular time and allocates power from the power sourcesto the step wave AC output based on the amount of power available fromeach of the power sources at the particular time.
 3. The step wave powerconverter according to claim 1, wherein the source management circuitrycomprises a plurality of high frequency converters tied to each powersource.
 4. The step wave power converter according to claim 3, whereinthe source management circuitry determines an amount of power that canbe contributed by each power source at a particular time and permitseach source to contribute up to a total amount of power requireddepending on a type of the power source and on the amount of power thatcan be contributed at that particular time by each power source.
 5. Astep way power converter comprising: a plurality of transformers, eachconfigured to receive DC voltage from one or more power sources andbeing further configured to supply a step for a step wave AC output; aplurality of bridge circuits for controlling DC voltage inputs from aplurality of DC sources into windings of the transformers to generatethe steps of the step wave AC output from the transformers; and whereinthe plurality of transformers comprise a plurality of three-phasetransformers, each three-phase transformer being configured to receiveDC voltage from one or more power sources and to supply a step for eachphase of a three-phase step wave AC output.
 6. The step wave powerconverter according to claim 5, further comprising: a pulse widthmodulator for controlling an input into a selected one of thetransformers to fine tune the step wave AC output in substantialconformance with an ideal AC waveform by ramping up or down the inputinto the selected one of the transformers to cause the output to moreclosely approximate an ideal AC waveform.
 7. The step wave powerconverter according to claim 5, wherein: each of the plurality of bridgecircuits comprising multiple gate pairs arranged in parallel, whereineach gate pair comprises two or more gates arranged in series; and eachbridge circuit being connected to one of the transformers, whereinopposite ends of a transformer winding are connected between gates inseparate gate pairs.
 8. The power converter of claim 7, wherein a firstset of transformer windings are arranged in a delta configuration. 9.The power converter of claim 8, wherein a first end of a first windingis connected between gates in a first one of the gate pairs and a secondend of the first winding is connected between gates of a third one ofthe gate pairs; wherein a first end of a second winding is electricallycouple to the second end of the first winding and a second end of thesecond winding is connected between gates in a second one of the gatepairs; and wherein a first end of a third winding is electricallycoupled to the second end of the second winding and a second end of thethird winding is electrically coupled to the first end of the firstwinding.
 10. The power converter of claim 8, wherein a second set ofwindings are arranged in a wye configuration.
 11. The power converter ofclaim 10, further comprising a phase management controller for enhancinga resolution of the three-phase step wave AC output by managingcharacteristics of the voltage transformation between the deltaconfiguration of the first set of windings and the wye configuration ofthe second set of windings.